1. Field
The aspects of the disclosed embodiments relate to a de-icing or defogging system for an optical instrument such as an image acquisition device. It also relates to an image acquisition device equipped with such a de-icing and/or defogging system.
The disclosed embodiments apply notably to a camera fitted to an aircraft.
2. Brief Description of Related Developments
It is known to equip airplanes with external cameras in a fixed position for the surveillance of a specific zone of the airplane and/or its environment. These cameras enable the pilot to visualize in real time vital or inaccessible parts of his aircraft such as the wings, undercarriage, cargo hold etc.
As an illustration, such a camera enables, in this way, to visualize precisely the position of the wheels on the runway and any obstacles when the airplane travels along the ground.
However, these cameras are subject to extreme conditions existing outside an airplane at the flying altitude. As an illustration, at 12,000 m altitude, the temperature outside the airplane approaches −50° C. Also, these cameras may be exposed to temperature ranges extending from −55° C. to +70° C. according to the flight phase.
These cameras typically have an image sensor and an objective that are placed inside a protective casing to protect them from ambient conditions, i.e. temperature and humidity.
However, air trapped inside the casing may contain a certain amount of water.
Now, it is observed that when the temperature outside the protective casing falls rapidly, this water rapidly condenses on the coldest part thereof that is often situated in the middle of the porthole or protective glass, placed in front of the optics of the image sensor.
The central part of image is then made unusable. This condensation may moreover cause the quality of the rest of the image generated in this way to deteriorate and in extreme cases make it totally unusable.
In addition, once this condensation has appeared, it may persist over a long period of time even when the conditions that created it no long come together.
Methods are known for a porthole defogging treatment but however these treatments may age with time and the porthole is then found to become opaque, making the image of the sensor blurred.
Finally, it is also known that when an aircraft flies above a certain altitude, droplets of water present in the atmosphere may, under certain conditions, accumulate in the form of frost on the external surfaces of the protective casing. These droplets then form a thickness of frost by accumulating on each other. This accretion of frost may make the sensor image totally unusable.
Once this frost layer is formed, and if no de-icing system is provided, this layer remains on the structure as long as the external temperature does not rise sufficiently to melt it.
The result is that a pilot may be deprived of visual access to some parts of the aircraft by reason of fogging or frost created by the accumulation of water particles on the protective glass or porthole of the camera normally used to visualize these parts.
It would therefore be valuable to have available an image acquisition device such as a video camera or a digital photographic apparatus, of which the structure prevents the formation of fogging inside or frost outside the protective casing.
Heating portholes are known in the state of the art that are made with electric wires connected to the porthole. However, these portholes are very costly and during maintenance of the acquisition device, these electric wires may be cut inadvertently during demounting, making the device ineffective.